1. Field of the Invention
The present invention relates to a semiconductor device incorporating a characteristic evaluating circuit.
2. Description of the Related Art
Generally, as time goes by, semiconductor devices deteriorate and break down. Therefore, in order to evaluate the characteristics of semiconductor devices, when new semiconductor devices are designed, developed and experimented with, characteristic evaluating circuits are incorporated therein to carry out various reliability tests. Only after the devices have passed the reliability tests, are the new semiconductor devices actually manufactured as products for shipping. The manufactured semiconductor devices are then assembled and mounted on a system board.
In spite of the semiconductor devices having passed the reliability tests, if one of the semiconductor devices on the system board deteriorates, the entire system board would break down.
In order to detect deterioration of the characteristics of semiconductor devices even after the semiconductor devices are assembled and mounted on a system board, a prior art semiconductor device is known where a characteristic evaluating circuit is introduced (see: JP-11-27128-A).
In the above-mentioned prior art semiconductor device, in a normal operation mode, a main circuit such as a logic circuit and a characteristic evaluating circuit are operated by the same clock signal, and in a characteristic evaluating mode, the characteristic evaluating circuit is electrically separated from the main circuit. This will be explained later in detail.
In the above-described prior art semiconductor device, however, since the main circuit and the characteristic evaluating circuit are operated by the same clock signal, when deterioration of the characteristic of the transistor of characteristic evaluating circuit is detected, the characteristics of the transistors of the main circuit also would deteriorate. Therefore, after the deterioration of the characteristic of the transistor of the characteristic evaluating circuit is detected, a system board on which this semiconductor device is mounted would break down.
Also, in the above-described prior art semiconductor device, in order to evaluate the characteristic evaluating circuit, a monitoring circuit such as a reference circuit for determining or not whether a detected threshold voltage is within a predetermined range, will be necessary. In addition, the characteristic evaluation circuit requires an output terminal. Therefore, this semiconductor device is disadvantageous in size.